Pulse laser apparatus, terahertz measuring apparatus, and terahertz tomographic apparatus

Abstract

A pulse laser apparatus includes a laser configured to generate a pulse of a laser beam, a fiber amplifier, and a pulse compressor. The fiber amplifier includes a rare-earth doped fiber that exhibits normal dispersion at a wavelength of the laser beam generated from the laser. The pulse laser apparatus further includes a unit configured to give a loss to energy portions in a wavelength region corresponding to a zero-dispersion wavelength of the rare-earth doped fiber and/or a wavelength region longer than the zero-dispersion wavelength within a wavelength spectrum of the laser beam having been chirped in the fiber amplifier.

1. A pulse laser apparatus comprising:
 a laser configured to generate a pulse of a laser beam; 
 a fiber amplifier configured to amplify the pulse from the laser; and 
 a pulse compressor configured to compress the pulse from the fiber amplifier, 
 wherein the fiber amplifier includes a rare-earth doped fiber that exhibits normal dispersion at a center wavelength of a spectrum of the pulse from the laser, and 
 wherein the fiber amplifier includes a unit configured to give a loss to energy portions in a wavelength region corresponding to a zero-dispersion wavelength of the rare-earth doped fiber and a wavelength region longer than the zero-dispersion wavelength within a spectrum of the pulse having been chirped and spread in the fiber amplifier. 
 
     
     
       2. The pulse laser apparatus according to  claim 1 , wherein the unit configured to give a loss to the energy portions in the zero-dispersion wavelength region and the wavelength region longer than the zero-dispersion wavelength is a wavelength filter. 
     
     
       3. The pulse laser apparatus according to  claim 1 , wherein the unit configured to give a loss to the energy portions in the zero-dispersion wavelength region and the wavelength region longer than the zero-dispersion wavelength generates a leakage loss in both the wavelength regions by forming a bent portion at least in a part of the rare-earth doped fiber. 
     
     
       4. The pulse laser apparatus according to  claim 1 , wherein the unit configured to give a loss to the energy portions in the zero-dispersion wavelength region and the wavelength region longer than the zero-dispersion wavelength is the rare-earth doped fiber having, at least in a part thereof, a W-shaped sectional refractive-index profile. 
     
     
       5. The pulse laser apparatus according to  claim 1 , wherein the unit configured to give a loss suppresses higher-order nonlinear effects generated during propagation of the laser beam through the rare-earth doped fiber by giving the loss to the energy portions in the zero-dispersion wavelength region and the wavelength region longer than the zero-dispersion wavelength. 
     
     
       6. The pulse laser apparatus according to  claim 1 , wherein the laser beam generated by the pulse laser apparatus has a pulse width of 20 fs or less and an average output of 200 mW or more. 
     
     
       7. The pulse laser apparatus according to  claim 5 , wherein the higher-order nonlinear effect to be suppressed is a phenomenon of four-wave mixing. 
     
     
       8. The pulse laser apparatus according to  claim 5 , wherein the higher-order nonlinear effect to be suppressed is induced Raman scattering. 
     
     
       9. The pulse laser apparatus according to  claim 3 , wherein a curvature of the bent portion is variable, and the pulse laser apparatus includes a unit configured to adjust the curvature of the bent portion while monitoring a waveform. 
     
     
       10. A terahertz pulse generating apparatus including:
 a photoconductive device or a nonlinear crystal; and 
 the pulse laser apparatus according to  claim 1 , 
 wherein a terahertz pulse is generated by irradiating the laser beam from the pulse laser apparatus to the photoconductive device or the nonlinear crystal. 
 
     
     
       11. A terahertz measuring apparatus including:
 the pulse laser apparatus according to  claim 1 ; and 
 a branch unit arranged to branch an optical output of the pulse laser apparatus into two parts, 
 wherein one part of the optical output is irradiated to a first photoconductive device or a first nonlinear crystal to generate a terahertz pulse, and the other part of the optical output is irradiated to a second photoconductive device or a second nonlinear crystal such that the second photoconductive device or the second nonlinear crystal operates as a detector, thus performing terahertz time domain spectroscopy in accordance with pump-probe measurement. 
 
     
     
       12. The terahertz measuring apparatus according to  claim 11 , wherein light irradiated to the second photoconductive device or the second nonlinear crystal is obtained by passing the laser beam output from the pulse laser apparatus through a higher-harmonic generator and taking light having passed through the higher-harmonic generator as the irradiated light. 
     
     
       13. A terahertz tomographic apparatus wherein internal tomographic image data of a specimen is obtained by measuring a reflected pulse from the specimen with the terahertz measuring apparatus according to  claim 11 , and an internal tomographic image is output to an output unit on the basis of the obtained data. 
     
     
       14. A terahertz tomographic apparatus wherein resolution in a direction of depth is 5 μm or less when internal tomographic image data of a specimen is obtained by measuring a reflected pulse from the specimen with the terahertz measuring apparatus according to  claim 11 . 
     
     
       15. The pulse laser apparatus according to  claim 1 , wherein the rare-earth doped fiber is an erbium doped fiber, a thulium doped fiber, or an ytterbium doped fiber. 
     
     
       16. A method of using the pulse laser apparatus according to  claim 1 , comprising irradiating the laser beam from the pulse laser apparatus to a photoconductive device or a nonlinear crystal to generate a terahertz pulse. 
     
     
       17. A method of using the pulse laser apparatus according to  claim 1 , comprising:
 branching an optical output of the pulse laser apparatus into two parts; 
 irradiating one part of the optical output to a first photoconductive device or a first nonlinear crystal to generate a terahertz pulse; and 
 irradiating the other part of the optical output to a second photoconductive device or a second nonlinear crystal such that the second photoconductive device or the second nonlinear crystal detects the terahertz pulse. 
 
     
     
       18. A pulse laser apparatus comprising:
 a laser configured to generate a pulse of a laser beam; 
 a fiber amplifier configured to amplify the pulse from the laser; and 
 a pulse compressor configured to compress the pulse from the fiber amplifier, 
 wherein the fiber amplifier includes a rare-earth doped fiber that exhibits normal dispersion at a center wavelength of a spectrum of the pulse from the laser, 
 wherein the fiber amplifier includes a unit configured to give a loss to energy portions in a wavelength region corresponding to a zero-dispersion wavelength of the rare-earth doped fiber and/or a wavelength region longer than the zero-dispersion wavelength within a spectrum of the pulse having been chirped and spread in the fiber amplifier, 
 wherein the unit configured to give a loss to the energy portions in the zero-dispersion wavelength region and/or the wavelength region longer than the zero-dispersion wavelength generates a leakage loss in both the wavelength regions by forming a bent portion at least in a part of the rare-earth doped fiber, and 
 wherein a curvature of the bent portion is variable, and the pulse laser apparatus includes a unit configured to adjust the curvature of the bent portion while monitoring a waveform.